"Contract won't or shouldn't change" seems unrealistically idealistic to me. Java migration - or any platform for that matter - is not just migrating its language features but also its ecosystem - runtime, VM, GC, frameworks.

There are many examples of migration problems for any platform - just have to google it. When information that can challenge our assumptions is just a google search away, I don't see why one should hold on to blind faith in idealistic assumptions. Here's one writeup of a system whose performance suffered simply by migrating to Java 8 without involving any code changes [1].

[1]: http://engineering.indeedblog.com/blog/2016/09/job-search-we...